This image cannot currently be displayed. 8/25/2016 Water Water Life is inextricably tied to water. Single most outstanding chemical property of water is its ability to form hydrogen bonds. Hydrogen Bonding Weak electrical attractions that form between the ends of polar molecules. This bond is between molecules, not within molecules. What s a polar molecule? How do they form electrical attractions? Why do we care? Polar and Nonpolar Molecules When the covalent bond forms between atoms with very unequal numbers of electrons, the molecule can become polar. Polar = Having ends with slightly different charges. Even though the whole molecule is neutral, the ends may have slightly different charges. Nonpolar= Does not have ends with different charges. H 2 0 : 1
Molecular Velcro Hydrogen bonds are like Velcro. Individual bonds are very weak, but many together are very strong. Water molecules are attracted to each other: Cohesion 2
Water molecules attracted to other kinds of molecules: Adhesion Cohesion water molecules stick to other water molecules by hydrogen bonding Adhesion water molecules stick to other polar molecules by hydrogen bonding 16 Hydrogen Bonds Very important inside your cells. Holds two halves of DNA together. Holds proteins together in their proper shapes in cells. Gives water its exceptional properties. 3
Properties of Water High specific heat & high heat of vaporization. Solid form is less dense than liquid form: 4
Water is a solvent Polar molecules and ions dissolve in water: hydrophilic. Water molecules gather around charged parts of other polar molecules, breaking them apart. Ex: salt & sugar. Form hydration shells around the ions. Helps move things across membranes by dissolving molecules. 26 Water organizes nonpolar molecules Nonpolar molecules are hydrophobic. Water repels them. Nonpolar molecules will then clump together: Hydrophobic exclusion. By doing this, hydrophobic molecules will assume particular shapes. Ex: formation of membranes. Water forms ions The covalent bonds of water sometimes break spontaneously. Hydrogen nucleus disassociates leaving its electron. This forms two ions: H 2 O OH + H + hydroxide ion hydrogen ion In pure water, this happens once out of every 550 million water molecules; and ions are equal. Concentration of these ions = acidity! Acids and Bases ph = partial hydrogen. The concentration of H+ ions. Any substance that dissolves in water to increasethe H+ (lowers ph) increasing the acidity is called an acid. Any substance that dissolves in water to decreasethe H+ (increases ph) decreases the acidity is called a base. 5
Cells like it neutral ph inside and outside cells (in multicellular organisms) is very close to neutral (ph 7). Too much change either way can damage or even kill cells. Homeostasis is important. Chemical reactions of life often produce acids and bases. Your body works to keep your cellular ph as neutral as possible, using buffers. 31 Buffers Substance that resists changes in ph. Act by : Releasing hydrogen ions when a base is added. Absorbing hydrogen ions when acid is added. Overall effect of keeping [H+] relatively constant (even with its OH- counterpart). Chemical reaction works in either direction. Most biological buffers consist of a pair of molecules, one an acid and one a base. Buffer Example: Your blood contains the acid/base pair of carbonic acid and bicarbonate. 33 Properties of Water High specific heat & high heat of vaporization. Large bodies of water moderate temperature. Solid form is less dense than liquid form. Ice floats. Water is a solvent. Polarity breaks apart other molecules bonds. Water organizes nonpolar molecules. Polarity repels nonpolar molecules and causes them to clump. Water forms ions. Acids & bases. 6
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